Method and apparatus for servicing counterbalanced lifting device

ABSTRACT

A counterbalanced lifting apparatus includes two weighted members suspended from cables which are trained over sheaves supported in an elevated position on a framework. One of the weighted members may be a bridge span and the other a counterweight. The first weighted member (bridge span) normally seats against a terminal surface while the second weighted member (counterweight) is normally suspended. During operational cycles, the relative positions of the weighted members reverse, and the counterweight may seat on cylinder-type buffers affixed to its underside. Some servicing operations require slack in the cables and this is achieved by seating the first weighted member against its terminal surface and then supporting jacking beams on a temporary support system installed directly beneath the cylindertype buffers of the second weighted member. Thereafter, a hydraulic pump is connected with the barrels of the cylinder-type buffers and hydraulic fluid is pumped into these barrels to cause the piston rods therein to extend against the jacking beams. This raises the second weighted member and thereby loosens the cables.

[ June 26, 1973 United States Patent [191 Newman ABSTRACT METHOD ANDAPPARATUS FOR G m m m an m Mm m WA C md n NA La L m A m n E & N N .n. Um 0 Rd C en v MOS V nmD Rve EEV SDhA UH WU Inc., Saint Louis, Mo.

Dec. 23, 1971 weighted member (bridge span) normally seats against aterminal surface while the second weighted member (counterweight) isnormally suspended. Durin [22] Filed:

g opera- [211 Appl' 211241 tional cycles, the relative positions of theweighted members reverse, and the counterweight may seat oncylinder-type buffers affixed to its und 14/42, 187/94 erside. Some ser-E01d 15/02 vicing operations require slack in the cables and this is 14/1, 42, 52, 66, achieved by seating the first weighted member against14/77, 36; 248/325, 364; 187/94, 71, 67; 254/178 its terminal surfaceand then supporting jacking beams on a temporary support systeminstalled directly beneath the cylinder-type buffers of the secondweighted [52] 11.8. [51] lnt. [58] Field of Search [56] References CitedUNITED STATES PATENTS 2,198,810 4/1940 member. Thereafter, a hydraulicpump is connected with the barrels of the cylinder-type buffers andhydraulic fluid is pumped into these barrels to cause the piston rodstherein to extend against the jacking beams. This raises the secondweighted member and thereby loosens the cables.

3,466,686 /1969 Allen 3,707,011 12/1972 Launay 392,753 11/1888 FOREIGNPATENTS OR APPLICATIONS 996,179 6/1965 Great 14/1 Primary ExaminerRoy D.Frazier Assistant Examiner-Thomas .l. Holko 14 Claims 9 Drawing FiguresAtlorney- Frederick M. Woodruffand Edward A.

Boeschenstein METHOD AND APPARATUS FOR SERVICING COUNTERBALANCED LIFTINGDEVICE BACKGROUND OF THE INVENTION This invention relates in general tocounterbalanced lifting devices and more particularly to a method andapparatus for servicing the same.

A conventional lift bridge has a pair of towers set on piers and a liftspan which extends between the piers. The span is usuallycounterbalanced against a massive counterweight in each tower sorelatively little power is required to lift the span. Thesecounterweights are connected to the span by cables which are trainedover sheaves at the top of the towers. The cables must be replaced atperiodic intervals and the sheave bearings may need to be pulled forservicing or repair. To replace the cables or to pull the sheavebearings, the cables must be unloaded and relaxed so that slack existsin them.

Presently, the cables are loosened or relieved of load by hoisting thecounterweights with hydraulic lift jacks which are positioned adjacentto the sheaves during the jacking operation. Each jack actually rests onajacking girder and lifts a jacking beam which is connected to thecounterweight through a tie rod. The jacking girders are all temporaryinstallations and must be emplaced at the top of the tower for thelifting operation to proceed. Moreover, since cables have a relativelylong life, the jacks are used infrequently and this leads to adeterioration of the seals and packing in them. Consequently, the jackssometimes become inoperable or fail in use. Also, when emplaced, thejacks are positioned closely opposite the inner sheave bearings whichmakes it extremely difficult to pull these bearings or to otherwiseservice them. In the same vein, most lift bridges have the hoistmachinery located at the tops of their towers adjacent to the sheavesfor driving the sheaves. Thus, in most lift bridges the tops of thetower are congested areas and the emplacement of the jacking equipmentto temporarily elevate the counterweight merely creates more congestionand compounds the servicing problem.

SUMMARY OF THE INVENTION One of the principal objects of the presentinvention is to simplify the lifting of counterweights on lift bridgesand the like to facilitate servicing of the cables and relatedequipment. Another object is to eliminate the need for emplacing jackingequipment at the top of the towers on such bridges when it is necessaryto loosen the cables or service bearings. A further object is to providea method and apparatus for simplifying the servicing of counterbalancedlifting devices having cylinder-type pneumatic buffers through dualutilization of existing components. These and other objects andadvantages will becomeapparent hereinafter.

The present invention is embodied in a method of loosening a flexibleconnector which is trained over a sheave and is connected to first'andsecond weighted members of a counterbalanced lifting apparatus. Thesecond weighted member carries cylinder-type pneumatic buffers. Themethod includes bringing the first weighted member against a terminalsurface, and thereafter placing an adequate load carrying jackingsurface under the cylinder-type buffer. Hydraulic pump means is thenconnected to the cylinder-type buffer to cause the piston and rod toextend and lift the second memher, thereby loosening the flexibleconnector. In addition, the invention resides in the apparatus topracticing the method. The invention also consists in the parts and inthe arrangements and combinations of parts hereinafter described andclaimed.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form partof the specification and wherein like numerals refer to like partswherever they occur:

FIG. 1 is a lift bridge on which the jacking system of the presentinvention is utilized;

FIG. 2 is an enlarged side elevational view of one of the towers on thebridge;

FIG. 3 is an enlarged front elevational view of the tower;

FIG. 4 is a plan view of the hoist machinery sheaves located in the topof the tower;

FIG. 5 is a sectional view of one of the cylinder-type buffers on theunderside of the counterweight;

FIG. 6 is a schematic view showing the pneumatic circuit connected withthe buffers;

FIG. 7 is a schematic view of the hydraulic circuit forming part of thepresent invention; and

FIGS. 8 and 9 are side elevational views of optional towers in whichvariations for providing the jacking surface are utilized.

DETAILED DESCRIPTION Referring now in detail to the drawings, 2designates a counterbalanced lift bridge (FIG. 1) including a pair ofconcrete piers 4, a lift span 6 bridging the piers 4, and towers 8positioned over the piers 4 and located at the ends of the lift span 6.Both towers 8 are the same, and consequently only the construction ofand the equipment housed in the right tower 8 will be discussed indetail.

The right tower 8 (FIGS. 2 and 3) has a framework 10 anchored on thepier 4 and extending upwardly therefrom. This framework 10 includes apair of front corner legs or columns 12 and a pair of rear corner legsor columns 14, and each of these corner columns 12 and 14 is formed froma series of upright girders positioned end to end. The two front columns12 are connected at the top thereof by a horizontal front girder l6 andbelow the front girder 16 they are further connected by a horizontalintermediate girder l8. Similarly, the rear columns 14 are connected attheir tops by a horizontal rear girder 20, and downwardly therefrom byanother intermediate girder 22 which is at the same elevation as theintermediate girder 18. The four columns 12 and 14 are further connectedby other horizontal and diagonal girders which act as bracing for thetower 8.

Extending between the front and rear girders 16 and 20 are four sheavegirders 26 which are arranged two pairs at each side of the tower 8. Thesheave girders 26 in turn support trunnion bearings 28, which arepreferably the cylindrical roller variety, and the pair of bearings 28for each pair of sheave girders 26 receive a single trunnion shaft 30.Each trunnion shaft 30 has a sheave 32 mounted on it between itsbearings 28 so that the sheaves 32 rotate between the sheave girders 26of each pair. The bearings 28 are positioned such that the sheaves 32project forwardly beyond the front columns 12. Each sheave 32 has aplurality of circumferential cable grooves 36 (FIG. 4) extending aroundit and may carry a large spur gear on sheave-drive type installations.

The sheave girders 26 also may support a machinery floor 40 on which thehoist machinery for the span 6 rests. That'mahinery is conventional andwill not be described in detail, other than to note that it includes(FIG. 4) drive motors 42 which are connected by means of drive shafts 44to a speed reducer 46. The speed reducer 46 delivers the power to acommon drive shaft 48 which extends along the back sides of the sheaves32 and has pinion gears 50 which mesh with the spur gears 34 on thesheaves 32. Brakes 52 are provided for retarding rotation of the driveshaft 44. The hoist machinery may also be located at the base of thetower 8 or on the lift span 6.

Trained over the sheaves 32 are cables 54 (FIGS. 2 and 3) which arearranged in two setsone at each sheave 32--and these cables 54 extenddownwardly in front of the front corner columns 12 and are attached tothe span 6 by means of cable connectors 56. Each cable set includes aplurality of cables 54 with each cable 54 being maintained in the properaxial position by a single circumferential cable groove 36 on the sheave32. On the opposite sides of the sheaves 32, the cables 54 dependthrough generally the center of the tower 8 and their ends are connectedby means of cable connectors 58 to a counterweight 60 located within thetower 8. The counterweight 60 weighs approximately half the weight ofthe span 6 and preferably is a large steel tank filled with concrete. Atits sides, the counterweight is provided with guide shoes 62 whichengage vertical guide rails 64 on the framework so that thecounterweight 60 is free to move vertically but not horizontally.

The span 6 is also provided with guide shoes 66 (FIG. 2) and these shoesare located at its ends and engage guide rails 68 on the two frontcorner columns 12. The span 6, when in its lowermost position, rests onbearing shoes 70 (FIG. 1) which are supported on the pier 4 in front ofthe tower 8. The bearing shoes 70 thus form a terminal surface for thespan 6.

Since the span 6 is counterbalanced by the counterweights 60 relativelylittle energy is required to raise the span 6 off of its bearing seat70. This'energy is supplied through the drive motors 42 which rotate thesheaves 32 at the tops of the towers 8. As the span 6 rises, thecounterweights 60 descend, the former following the guide rails 68, andthe latter the guide rails While the span 6 rests on the bearing seat 70when in its lowermost position, the counterweights 60 when they come totheir lowermost position, rest against pads 72 (FIG. 1) which areelevated above the seats to approximately the level of the bridge deckby posts 74 extending upwardly from the piers 4.

The counterweight 60 does not directly engage its pads 72, but on thecontrary, is provided with a pair of cylinder-type pneumatic buffers 80which cushion the impact with the pads 72. Likewise, the span 6 on itsunderside is also provided with cylinder-type pneumatic buffers 80 whichretard the downward movement of the span 6 prior to its engagement withthe bearing seats 70 so that the span is eased onto the bearing seats70. The buffers 80 therefore avoid inducing bounce or shock in thestructure.

The buffers 80 for each counterweight 60 are disposed at the sides ofthe counterweight 60 directly beneath the cable connections 58, and areaccordingly centered with respect to its front and rear faces. Eachbuffer includes (FIG. 5) a barrel 82 having a cylindrical sidewall whichis closed at its upper end by a head or cap 84 and at its lower end by ahead 86. The cylindrical side wall is clamped between the heads 84 and86 to bolts 88, while the upper head 84 is further bolted or otherwisefastened against the underside of the counterweight 60. The head 84 hasa port 90 and a passageway 92 which provides communication between theport 90 and the interior of the barrel 82. The lower head 86, on theother hand, has a vent port 94 which communicates with the interior ofthe barrel 82. The lower head 86 also receives a piston rod 96 andcarries a guide sleeve 98 which wipes the outer surface of the rod 96.Within the barrel 82 the piston rod 96 is fastened to a piston 100having piston rings 102 which wipe and seal against the inwardlypresented surface of the barrel 82. The lower ends of the piston rods 94are fitted with striking heads 104 which align with the pads 72 on thepier 4.

The pressure port 90 of each buffer 80 is connected (FIG. 6) to an airdischarge line 108 and the two discharge lines 108 empty into a commonchoke line 110 containing a choke valve 112 and a pressure relief valve114, the former of which limits,the rate at which air may be expelledfrom the barrels 82. The choke valve 112 should be adjustable andpreferably takes the form of a needle valve. Each discharge line 108intermediate its ends communicates with a check valve 116 which isoriented such that it allows air to flow into the discharge line 108,that is toward the buffer barrels 82, but not in the opposite direction.The vent port 94 of each buffer 80 is connected to a vent line 118containing a check valve 120 which is oriented to allow air to flowtoward the buffer barrel 82, but not in the opposite direction.Intermediate the vent port 94 and the check valve 120, a choke valve 122communicates with the vent line 118. The choke valve 122 likewise isadjustable and is preferably a needle valve.

When the counterweight 60 is in its elevated position, as it normallyis, the pistons 100 of the buffer 80 are in their lowermost positions,that is against the apertured head 86, and the piston rods 96 areextended. Once the drive motors 42 are energized, the span 6 rises,while the counterweight 60 descends. As the span 6 nears its uppermostposition the speed of the drive motors 42 is reduced and in the lastincrement of movement the striking heads 104 at the ends of the pistonrods 96 engage the fixed pads 72. Continued movement of thecounterweight 60 causes the pistons 100 to move through their respectivebarrels 82 and compress the air trapped in the barrels 82. Thecompressed air is expelled through the discharge lines 108 and chokeline 110, but since the choke valve 112- permits only a restricted flowof air, air compresses within the barrels 82, causing the pistons 100 tomove progressively more slowly through their barrels 82, and thisprevents the counterweight 60 from coming to an abrupt halt on the pads72. The check valves 120 allow air to enter the portions of the barrels82 below the pistons 100 so vacuums do not develop in those portions ofthe barrels 82.

When the span 6 is lowered by reversing the drive motor 42, thecounterweight 60 rises. As the counterweight 60 lifts away from the pads72, the weight of the pistons 100 and piston rods 96 in the barrels 82of buffers 80 causes these pistons 100 to pass downwardly in theirbarrels 82 and force air out of the barrels 82 through the vent ports94. Again the movement is impeded by the choking effect the choke valves120 have on the airstream. A vacuum is not drawn above the pistons 100since the check valves 116 allow air to flow into the upper portions ofthe barrels 82 through the discharge lines 108.

The buffers 80 on the span 6 operate in the same manner.

To replace the cables 54 or to pull the trunnion bearings 28 it isnecessary to relax the tension in the cables 54 to the point that someslack exists in them, and this is achieved by converting the pneumaticbuffers 80 into hydraulic jacks. In particular, with the span 6 in itslowermost position, that is resting on the pads 70, the buffers 80 onthe counterweight 60 are disconnected from the discharge lines 108 andthe vent lines 118, and are instead connected into a hydraulic jackingcircuit 130 (FIG. 7). That circuit 130 includes a reservoir 132containing hydraulic fluid and a pump 134 having its suction portconnected with the reservoir 132. The discharge port of the pump 134 isconnected to a 3 position bidirectional valve 136, through a supply line138, and this valve is further connected to the reservoir 132 through areturn line 140. In one position, the valve 136 directs pressurizedfluid from the supply line 138 to a feed line 142 which is connected tothe pressure ports 90 at the upper ends of the buffer barrel 82. Inanother position the valve 136 directs fluid from the high pressuresupply line 138 to a feed line 144 which is connected to the vent ports94 at the lowerv ends of the buffer barrels 82. Of course, when thelines 138 and 142 are in communication, the feed lines 144 and returnline 140 communicate through the valve 136, and likewise when the lines138 and 144 are in communication, the feed line 142 and return line 140communicate through the valve 136. In its third position, the valve 136places the supply line 138 and return line 140 in communication so thatfluid is merely circulated back to the reservoir 132 withoutpressurizing either of the feed lines 142 or 144.

The feed line 142 possesses a common branch containing a manualauxiliary shut-off valve 146 and a pair of individual branches leadingto the ports 90 of each individual buffer barrel 82, and theseindividual branches contain manual regulator valves 148 for controllingthe distribution of hydraulic fluid to the two barrels 82. At thejuncture of its common and individual branches, the feed line 142 has ableed valve 150 through which air is purged from the hydraulic circuit130. Connected between the supply line 138 and the return line 140 is apressure relief valve 152.

After the counterweight 60 is in its uppermost position and thehydraulic jacking circuit 130 is connected to the buffers 80 at thepressure and vent ports 90 and 94 thereof (FIG. 7); the pump 134 isenergized and the valve 136 is set to direct the pressurized fluid intothe line 144. This fluid enters the lower end of the buffer barrels 82through the vent ports 94 and causes the piston rods 96 to retract intothe barrels 82. Next, jacking beams 160 (FIGS. 2 and 3) are placedacross and supported on the horizontal intermediate girders 18 and 22 ofthe tower framework 10, and these beams are positioned such that theyare directly beneath the retracted piston rods 96 of the buffers 80.These jacking beams 160 provide a reaction surface against which thebuffers 80 act when they are converted to jacks, and this reactionsurface may be elevated slightly by placing blocking 162 between thebeams 1 60 and the striking heads 104 of the piston rods 96.

Once the jacking beams 160 and blocking 162 are in place, the valve 136is moved to direct the high pressure fluid from the pump 134 and supplyline 138 to the feed line 142 from which it is introduced into the upperends of the barrels 82 through the pressure ports 90. This fluid actsagainst the pistons and lifts the barrels 82 along with the entirecounterweight 60. As the counterweight 60 moves upwardly support columns164 and shims 166 are placed between it and the jacking beam to preventthe counterweight 60 from falling in the event ofa hydraulic blowout. Ifit is necessary to reblock the jacking arrangement, the support columns164 and shims 166 are used to support the counterweight 60 while thepiston rods 96 are again retracted and additional blocking 162 is placedbetween them and the beams 160. Of course, as the counterweight 60 iselevated by the buffers 80, the tension in the cables 54 lessens andthey become slack.

In lieu of supporting the jacking beams 160 on both the horizontalintermediate girders 18 and 22, the front ends of the beams 160 may besupported on the front intermediate girders 18 while the rear ends aresuspended from the sheave girders 26 by hanger rods 170 (FIG. 8).Indeed, both the front and rear ends of the jacking beams 160 may besuspended by hanger rods 170 from the sheave girders 26 (FIG. 9). Ineither case, stabilizing struts 172 extend between the framework 10 andthe beams 160. The hanger rods 170 are more suitable for towers in whichthe rear corner columns 14 are not vertical or for towers which do nothave horizontal intermediate girders 18 and 22 or else have such girdersin a position not suitable for jacking.

Since the jacking equipment is all positioned beneath the counterweight60, sufficient room exists at the top of the tower 8 for servicing thesheaves 32 and its bearings 28. Also, no additional jacks are necessary.Since the buffers 80 are used and serviced frequently, the chances ofthe deterioration of the equipment is greatly reduced, and this lessensthe chances of the buffers 80 being inoperative or failing when they areemployed as jacks.

This invention is intended to cover all changes and modifications of theexample of the invention herein chosen for purposes of the disclosurewhich do not constitute departures from the spirit and scope of theinvention.

What is claimed is:

1. A method of servicing a counterbalanced lifting apparatus includingfirst and second weighted members suspended from a flexible connectorwhich is trained over a sheave supported on a frame and wherein thefirst weighted member normally rests on a fixed terminal surface and thesecond weighted member carries a cylinder-type pneumatic buffer having abarrel and a piston and piston rod shiftable in the barrel; said methodcomprising: seating the first member on the terminal surface; placing areaction surface under the cylinder-type buffer of the second member;and pumping hydraulic fluid into the barrel of the cylinder-type bufferto cause the piston rod therein to extend against the reaction surfaceand lift the second weighted member, whereby slack is imparted to theflexible connector.

2. A method according to claim 1 wherein the second member is elevatedwhen the first member is seated against the terminal surface and thereaction surface is carried by a jacking member; and wherein the step ofplacing a reaction surface under the buffer includes supporting thejacking member on the frame.

3. A method according to claim 1 wherein the reaction surface is carriedby a jacking beam; and wherein the step of placing the reaction surfaceunder the buffer includes resting both ends of the beam on the frame.

4. A method according to claim 1 wherein the reaction surface is carriedby a jacking beam; and wherein the step of placing the reaction surfaceunder the buffer includes resting one end of the beam on the frame andsuspending the other end of the beam from the frame.

5. A method according to claim 1 wherein the reaction surface is carriedby a jacking beam; and wherein the step of placing the reaction surfaceunder the buffer includes suspending both ends of the beam from theframe.

6. A method according to claim 2 and further characterized by pumpinghydraulic fluid into the barrel of the cylinder-type buffer to cause thepiston rod therein to retract prior to supporting the jacking member onthe frame.

7. A method according to claim 6 and further characterized by placingshims between the jacking member and the first weighted member as thecylinder-type buffer expands.

8. In a counterbalanced lifting apparatus including first and secondweighted members suspended from a flexible elongated connector which istrained over a sheave supported on a frame, and wherein the secondweighted member carries a cylinder-type pneumatic buffer having a barreland a piston and piston rod shiftable in the barrel and the firstweighted member normally rests on a fixed terminal surface; theimprovement comprising a jacking member supported on the frame beneaththe pneumatic buffer when the first weighted member is seated againsttheterminal surface, the jacking member being removable from the path ofthe buffer so as not to interfere with the normal operation of thelifting apparatus, and hydraulic pump means connected to thecylinder-type buffer to pump pressurized fluid into the end of thebarrel which causes the piston rod to extend, whereby the cylinder-typebuffer expands between the jacking member and second weighted member andlifts the second weighted member, whereby slack is imparted to thecable.

9. The structure according to claim 8 wherein the hydraulic pump meansis also connected to the cylindertype buffer to pump pressurized fluidinto the opposite end of the barrel to cause the piston rod to retract,whereby the cylinder type buffer will contract so that the jackingmember may be placed under the buffer.

10. The structure according to claim 9 wherein the hydraulic pump meansincludes a valve for directing hydraulic fluid to either end of thebarrel on the cylinder type buffer.

11. The structure according to claim 8 wherein the jacking member is abeam, both ends of which rest on the frame.

12. The structure according to claim 8 wherein the jacking member is abeam, one end of which rests on the frame; and wherein a hanger rod isconnected between the frame and the opposite end of the beam forsuspending that opposite end of the beam from the frame.

13. The structure according to claim 8 wherein the jacking member is abeam; and wherein hanger rods are connected between the frame and bothends of the beam for suspending the beam from the frame.

14. The structure according to claim 9 wherein the first weighted memberis a bridge span and the second weighted member is a counterweight.

1. A method of servicing a counterbalanced lifting apparatus includingfirst and second weighted members suspended from a flexible connectorwhich is trained over a sheave supported on a frame and wherein thefirst weighted member normally rests on a fixed terminal surface and thesecond weighted member carries a cylinder-type pneumatic buffer having abarrel and a piston and piston rod shiftable in the barrel; said methodcomprising: seating the first member on the terminal surface; placing areaction surface under the cylinder-type buffer of the second member;and pumping hydraulic fluid into the barrel of the cylinder-type bufferto cause the piston rod therein to extend against the reaction surfaceand lift the second weighted member, whereby slack is imparted to theflexible connector.
 2. A method according to claim 1 wherein the secondmember is elevated when the first member is seated against the terminalsurface and the reaction surface is carried by a jacking member; andwherein the step of placing a reaction surface under the buffer includessupporting the jacking member on the frame.
 3. A method according toclaim 1 wherein the reaction surface is carried by a jacking beam; andwherein the step of placing the reaction surface under the bufferincludes resting both ends of the beam on the frame.
 4. A methodaccording to claim 1 wherein the reaction surface is carried by ajacking beam; and wherein the step of placing the reaction surface underthe buffer includes resting one end of the beam on the frame andsuspending the other end of the beam from the frame.
 5. A methodaccording to claim 1 wherein the reaction surface is carried by ajacking beam; and wherein the step of placing the reaction surface underthe buffer includes suspending both ends of the beam from the frame. 6.A method according to claim 2 and further characterized by pumpinghydraulic fluid into the barrel of the cylinder-type buffer to cause thepiston rod therein to retract prior to supporting the jacking member onthe frame.
 7. A method according to claim 6 and further characterized byplacing shims between the jacking member and the first weighted memberas the cylinder-type buffer expands.
 8. In a counterbalanced liftingapparatus including first and second weighted members suspended from aflexible elongated connector which is trained over a sheave supported ona frame, and wherein the second weighted member carries a cylinder-typepneumatic buffer having a barrel and a piston and piston rod shiftablein the barrel and the first weighted member normally rests on a fixedterminal surface; the improvement comprising a jacking member supportedon the frame beneath the pneumatic bufFer when the first weighted memberis seated against the terminal surface, the jacking member beingremovable from the path of the buffer so as not to interfere with thenormal operation of the lifting apparatus, and hydraulic pump meansconnected to the cylinder-type buffer to pump pressurized fluid into theend of the barrel which causes the piston rod to extend, whereby thecylinder-type buffer expands between the jacking member and secondweighted member and lifts the second weighted member, whereby slack isimparted to the cable.
 9. The structure according to claim 8 wherein thehydraulic pump means is also connected to the cylinder-type buffer topump pressurized fluid into the opposite end of the barrel to cause thepiston rod to retract, whereby the cylinder type buffer will contract sothat the jacking member may be placed under the buffer.
 10. Thestructure according to claim 9 wherein the hydraulic pump means includesa valve for directing hydraulic fluid to either end of the barrel on thecylinder type buffer.
 11. The structure according to claim 8 wherein thejacking member is a beam, both ends of which rest on the frame.
 12. Thestructure according to claim 8 wherein the jacking member is a beam, oneend of which rests on the frame; and wherein a hanger rod is connectedbetween the frame and the opposite end of the beam for suspending thatopposite end of the beam from the frame.
 13. The structure according toclaim 8 wherein the jacking member is a beam; and wherein hanger rodsare connected between the frame and both ends of the beam for suspendingthe beam from the frame.
 14. The structure according to claim 9 whereinthe first weighted member is a bridge span and the second weightedmember is a counterweight.